Rotating electrical machine

ABSTRACT

In a rotating electrical machine a stator winding is formed of flexible cables inserted in an open stator in the stator core. The stator slots extend from a radially outermost yoke part of the stator core radially inwards towards the rotor, the stator slots being separated by teeth. The stator slots are shaped as double slots with shoulders arranged in the slot walls such that each cable in the winding is secured between the previous cable in the stator slot and, seen from the slot opening, the next shoulder situated outside that particular cable. In a method for manufacturing such a winding the first cable is inserted through the opening of the stator slot and pressed firmly into a cable slot arranged at the bottom of the stator slot and designed to retain the cable when it has been pressed in. A second cable is inserted through the opening of the stator slot and secured between this first cable and an opposing securing shoulder in the wall of the slot, after which a third cable is inserted through the opening in the stator slot and secured between the second cable and a securing shoulder in the slot wall opposite the second cable, and so on.

TECHNICAL FIELD

The present invention relates to a rotating electrical machine with stator winding formed of flexible cable inserted in stator slots in the stator core, which stator slots extend from a radially outermost yoke part of the stator core radially inwards towards the rotor, and which stator slots are separated by teeth. The invention also relates to a method for manufacturing such a stator winding.

BACKGROUND ART

Electrical machines of the type mentioned above are described, for instance, in international patent application WO 97/45919. The cables forming the stator winding are suitably high-voltage cables of substantially the same type as those used for power distribution, i.e. PEX cables (PEX=cross-linked polyethylene). However, contrary to power distribution cable, the winding has no metal sheath which normally surrounds such a power distribution cable. The cable thus comprises substantially only the electrical conductor and at least one semiconducting layer on each side of the insulating layer. These machines can be built for extremely high voltages, 800 kV and higher, and for extremely high powers, up to 1500 MW.

When manufacturing the winding for such a stator, the stator is usually provided with mainly closed slots, the cable being drawn axially through the slots. In electrical machines intended for these extremely high voltages, several hundred kilovolt, the total length of cable in the stator winding will be considerable, up to several kilometers per phase. In such electrical machines the slots for the stator winding in the stator core must also be made very deep to ensure sufficient space for the required number of winding turns in the slot, and this in turn leads to the teeth that separate the slots becoming very long. Furthermore, it is essential for the function of these machines that the outermost semiconducting layer of the cable is not damaged during winding of the machine, since this layer is vital for enclosing the electric field in the winding.

In the case of machines intended for lower power, up to 30–40 MW, and lower voltages, to which the present invention relates, the stator winding is composed of a cable with considerably smaller diameter than the winding in a machine for high voltage and high power. In a generator for wind power plants of the type described, for instance, in international patent applications SE99/00943 and SE99/00944 and in published international application WO99/28919, the generator is driven directly and operates at very low speed, 10–25 rpm, which in turn means that it must be designed with a large number of poles. Many poles result in large diameter and thus high weight.

DISCLOSURE OF THE INVENTION

The object of the present invention is to eliminate the problems discussed above of high weight of the stator and extensive cable drawing therefor, and offer simplified manufacture of the type in question of electrical machine while maintaining an unbroken outer semiconducting layer for the winding.

This object is achieved with a rotating electrical machine of the type described in the introduction, with the characterizing features defined in claim 1.

Designing the stator core with open stator slots allows the stator winding to be produced by inserting the cable in the slots in radial direction. The cable need not thus be drawn axially through the slots. Furthermore by designing the slots as defined in claim 1, each new cable in the winding is secured between the previous cable in the slot and the next shoulder situated outside that particular cable. The cables are thus self-securing. Manufacture of the winding is greatly facilitated by each new cable being firmly clamped during the winding process between the previous cable and the shoulder immediately outside the cable. Furthermore, by shaping the slots in the stator core as double slots, substantially twice as many cables can be fitted into a stator slot of a certain depth than in a single slot. The slots need not therefore be as deep, resulting in shorter teeth, with less tendency to vibrate, between the slots as well as a total weight reduction for the stator core.

In accordance with an advantageous embodiment of the machine according to the invention, the stator slots and shoulders are so dimensioned that the distance between the cable inserted in the stator slot and the opposite shoulder is somewhat less, preferably some tenth of a millimeter less, than the diameter of the cable. For the securing of the cable mentioned above to function, said distance must on the one hand be somewhat less than the cable diameter. However it is of the utmost importance that the cable is not subjected to any appreciable deformation during production of the stator winding, which might lead to disastrous field concentrations, and the difference between said distance and the cable diameter should, on the other hand, be as small as possible.

In accordance with another advantageous embodiment of the machine according to the invention, an open, circular cable slot is shaped in the slot wall at the bottom of the stator slot, which cable slot has a radius corresponding substantially to the radius of the cable, and an opening that is somewhat less than 180° in order to secure the first cable in the stator slot in this cable slot. Shaping the cable opening somewhat less than 180° will ensure that the cable will be retained in the cable slot after having been pressed into the slot. However, it is important that the slot opening is sufficiently large for the cable not to be deformed when it is pressed in through the opening, which might result in the above-mentioned consequences.

In accordance with other advantageous embodiments of the machine according to the invention a profile of elastic material is inserted in the spaces between adjacent cables in the stator slot, or a sheet of elastic material is inserted between the cables in the two layers of cable in the double slot. The profile is dimensioned to absorb variations in the diameter of the cables and to keep the cable constantly pressed against the wall of the slot, and in equivalent manner the thickness of the sheet is adapted to absorb variations in the diameter of the cables and to keep the cable constantly pressed against the wall of the slot. The profile and the sheet can thus take up variations in the diameter of the cable resulting from settling, thermal expansion and deviations in the dimensions of the cable, as well as ensuring good cooling of the cable against the slot wall, and avoiding vibrations and wear damage in the cable.

In accordance with yet another advantageous embodiment of the machine according to the invention the cable comprises an insulation system surrounding a conducting core, said system comprising two semiconducting layers applied one on each side of a solid lay of insulation, the semiconducting layers constituting substantially equipotential surfaces. The electrical field is thus confined and the outer surface of the cable can be kept substantially at earth potential.

In accordance with still another advantageous embodiment of the machine according to the invention the solid insulation and its surrounding semiconducting layers have an electrical insulating strength exceeding 3 kV/mm, preferably exceeding 5 kV/mm. The dimensions of the winding can thus be kept down, despite high voltages in the winding, which in turn contributes to keeping the total dimensions of the machine down.

DESCRIPTION OF THE DRAWINGS

To explain the invention more fully, embodiments selected by way of example will now be described with reference to the accompanying drawings in which

FIG. 1 shows a cross-sectional view of a cable suitable for use in the cable-wound machine in accordance with the invention,

FIG. 2 shows a part of a stator lamination in a machine in accordance with the present invention, and

FIGS. 3–5 show various examples of insertion of the cable in the slots of the stator core of the machine in accordance with the invention.

DESCRIPTION OF EMBODIMENTS

The invention relates to a rotating electrical machine of cable-wound type. The cable 1 forming the stator winding is shown in cross section in FIG. 1 and constitutes a high-voltage cable of substantially the same type as those used for power distribution, i.e. PEX cables. The high-voltage cable 1 comprises a conducting core with a plurality of strands 2. The conductor is surrounded by an insulation system having two semiconducting layers 3, 5 applied one of each side of a solid layer of insulation 4. The cable is flexible and the semiconducting layers 3, 5 comprise substantially equipotential surfaces. This enables the electrical field to be confined so that the outer surface of the cable 1 can be kept at substantially earth potential. The solid insulation 4 and its semiconducting layers 3, 5 enclosing it have an electrical insulating strength exceeding 3 kV/mm, preferably exceeding 5 kV/mm.

The cable is thus eminently suitable for use as winding in a stator core for high voltages, while still maintaining control of the electrical field and without risk of destructive electrical partial discharges, PD, occurring.

FIG. 2 shows a part of a stator lamination in the stator core in accordance with the invention. Double slots 6 have been produced in the lamination, extending from a radially outermost yoke part 8 of the stator lamination radially inwards towards the rotor which shall be arranged in front of the openings of the slots. A cable slot 10 is shaped at the bottom of the stator slot, which cable slot has a radius corresponding substantially to the radius of the cable, so that the cable will achieve good contact with the wall of the slot. The slot opening is somewhat less than 180°. This slot 10 is intended to receive the first cable in the double slot 6 and, since the slot opening is somewhat less than 180°, the cable will be retained in this slot 10 when it has been pressed in. Shoulders 12 and 14, respectively, are arranged in the opposing walls of each slot. With the aid of these shoulders 12, 14, which are rounded with a radius of curvature that is somewhat less than the corresponding radius of the cable so that adjacent cables are in close contact with the curvature of the shoulders, each new cable inserted into the double slot 6 can be secured between a shoulder and the previous cable in the slot 6.

The double slots 6 are separated by teeth 16. Holes 18 are shown in the teeth 16 and in the yoke part of the lamination, to allow the stator laminations to be bolted together when manufacturing the stator core.

FIG. 3 illustrates a double slot of the type shown in FIG. 2, with winding cable inserted. The first cable 20 is inserted in the cable slot at the bottom of the stator slot and secured in this cable slot as described in conjunction with FIG. 2. Next cable 22 is then secured in the stator slot upon its insertion between the cable 20 and the shoulder 24 situated next outside the cable 22. Stator slot and shoulder are so dimensioned that the distance between the cable 20 and the opposite shoulder 24 is somewhat less than the diameter of the cable 22. This distance may typically be some tenth of a millimeter less than the cable diameter. Upon insertion into the stator slot the next cable 26 is then secured between the previous cable 22 and the shoulder 28 situated next outside it, and so on.

To firmly press the cables 20, 22, 26 against the wall of the slot, which is necessary in order obtain satisfactory cooling and avoid any vibrations, in the embodiment shown in FIG. 4 a profile 30 of elastic material is placed in the triangular spaces between adjacent cables. The profile 30 may be a thick-walled tube or a homogenous strip of silicon rubber or elastic cellular rubber. The profile 30 is dimensioned to absorb variations in the diameter of the cables 20, 22, 26 caused by settling, thermal expansion or deviations in dimension at manufacture.

Instead of inserting profiles as illustrated in FIG. 4, a sheet 32 of elastic material such as cellular rubber may be placed between the two layers of cables 20, 26 . . . and 22 . . . respectively, in the double slot, see FIG. 5. The thickness of the sheet 32 is adapted to firmly press the cables against the slot wall in the same way as the profiles 30, in order to achieve efficient cooling and avoid vibration which can result in wear damage, and also to take up variations in the cable diameter as mentioned above. In this context it may be mentioned that the cable described has a diameter of between 16 and 25 mm and is dimensioned for a voltage level of between 10 and 50 kV. The rotating electrical machine described is suitable as a directly driven generator in a wind power plant, with a power exceeding 1 MW, preferably exceeding 1.5 MW, and particularly between 3 and 6 MW. However, the rotating electrical machine described is also suitable as a motor and is then designed within the scope of the invention for considerably higher speeds than the directly driven wind power generator where the speed of the wind power rotor is a limiting factor for the speed of the generator. The machine can also be driven with considerably higher power in motor operation. 

1. A rotating electrical machine comprising: a stator winding formed of flexible cable inserted in stator slots in a stator core of the stator, which stator slots extend from a radially outermost yoke part of the stator core radially inwards towards the rotor, and which stator slots are separated by teeth, wherein said stator slots are shaped as open double slots with shoulders arranged in slot walls such that a next cable turn in the stator winding is secured between a previous cable turn in the stator slot and, seen from a slot opening, a next shoulder situated outside the next cable turn.
 2. A machine as claimed in claim 1, wherein the stator slot and the shoulders are dimensioned that a distance between the cable inserted in the stator slot and an opposite shoulder is less than a diameter of the cable.
 3. A machine as claimed in claim 1, wherein the shoulders are rounded with a radius of curvature less than a corresponding radius of the cable.
 4. A machine as claimed in claim 1, wherein an open, circular cable slot is shaped in at least one of the slot walls at a bottom of a respective stator slot, which cable slot has a radius corresponding substantially to a radius of the cable, and an opening less than 180° to secure the first winding cable in the stator slot in this cable slot.
 5. A machine as claimed in claim 1, wherein a profile of elastic material is inserted in spaces between adjacent cables in the stator slots.
 6. A machine as claimed in claim 5, wherein the profile comprises a thick-walled tube.
 7. A machine as claimed in claim 5, wherein the profile comprises a homogenous strip.
 8. A machine as claimed in claim 5, wherein the profile comprises silicon rubber.
 9. A machine as claimed in claim 5, wherein the profile comprises elastic cellular rubber.
 10. A machine as claimed in claim 5, wherein the profile is dimensioned to absorb variations in a diameter of the cables and to keep the cable constantly pressed against the wall of the slot.
 11. A machine as claimed in claim 1, wherein a sheet of elastic material is inserted between the cables in two layers of cable in the double slot.
 12. A machine as claimed in claim 11, wherein the sheet comprises cellular rubber.
 13. A machine as claimed in claim 11, wherein a thickness of the sheet is configured to absorb variations in a diameter of the cables and to keep the cable constantly pressed against the wall of the slot.
 14. A machine as claimed in claim 1, wherein the cable comprises an insulation system surrounding a conducting core, said insulation system comprising two semiconducting layers applied one on each side of a solid layer of insulation, the semiconducting layers forming essentially equipotential surfaces.
 15. A machine as claimed in claim 14, wherein the solid insulation and surrounding semiconducting layers have an electrical insulating strength exceeding 3 kv/mm. 